The Core D Mouse Acquisition, Breeding and Care program will provide support for Projects 1, 2, 3 and 4. The core will provide the following services for the projects. 1. Rapid backcross of the CTLA4-null locus from a mixture of 129 x B6 to the B6.RAG-(-/-) background. 2. Centralized ordering and maintenance of all strains of mice needed for the program to ensure the most economical use of resources. 3. Production ofhu-PBL-SCID mice with EBV lymphoma for experiments in Project 1.